A star-based method for precise wavelength calibration of the Chinese Space Station Telescope(CSST) slitless spectroscopic survey

The Chinese Space Station Telescope(CSST)spectroscopic survey aims to deliver high-quality low-resolution(R>200)slitless spectra for hundreds of millions of targets down to a limiting magnitude of about 21 mag,distributed within a large survey area(17500 deg2)and covering a wide wavelength range(255-1000 nm by three bands GU,GV,and GI).As slitless spectroscopy precludes the usage of wavelength calibration lamps,wavelength calibration is one of the most challenging issues in the reduction of slitless spectra,yet it plays a key role in measuring precise radial velocities of stars and redshifts of galaxies.In this work,we propose a star-based method that can monitor and correct for possible errors in the CSST wavelength calibration using normal scientific observations,taking advantage of the facts that(ⅰ)there are about ten million stars with reliable radial velocities now available thanks to spectroscopic surveys like LAMOST,(ⅱ)the large field of view of CSST enables efficient observations of such stars in a short period of time,and(ⅲ)radial velocities of such stars can be reliably measured using only a narrow segment of CSST spectra.We demonstrate that it is possible to achieve a wavelength calibration precision of a few km s^(-1) for the GU band,and about 10 to 20 kms^(-1) for the GV and GI bands,with only a few hundred velocity standard stars.Implementations of the method to other surveys are also discussed.

Precision of the Chinese Space Station Telescope(CSST)stellar radial velocities

The Chinese Space Station Telescope(CSST)spectroscopic survey plans to deliver high-quality low-resolution(R>200)slitless spectra for hundreds of millions of targets down to a limiting magnitude of about 21 mag,covering a large survey area(17500 deg^(2))and a wide wavelength range(255-1000 nm by three bands GU,GV,and GI).In this work,we use empirical spectra of the Next Generation Spectral Library to simulate the CSST stellar spectra at R=250,and investigate their capabilities in measuring radial velocities.We find that velocity uncertainties depend strongly on effective temperature,weakly on metallicity for only FGK stars,and hardly on surface gravity.It is possible to deliver stellar radial velocities to a precision of about 3 kms^(-1)for AFGKM stars,and about 10 km s^(-1)for OB stars,at signal-to-noise ratio(SNR)of 100.Velocity uncertainties using single GU/GV/GI band spectra are also explored.Given the same SNR,the GU band performs best,the GV band the second best,and then the GI band.The effects of spectral normalization and the imperfect template on velocity measurements are investigated and found to be very weak.The uncertainties caused by wavelength calibration are considered and found to be moderate.Given the possible precision of radial velocities,the CSST spectroscopic survey can enable interesting science such as searching for hyper-velocity stars.Limitations of our results are also discussed.